During night driving, the vision of a motor vehicle driver may be impaired by headlight glare reflected into his or her eyes by the vehicle's rearview or side mirrors. To alleviate this problem, conventional rearview mirrors in modern automobiles utilize prismatic methods and associated mechanisms to change the mirror from a high reflectance state to a low reflectance or dimmed state. In such a prior art rearview mirror, the dimmed state is achieved under night conditions by pointing a high reflectance surface of the mirror at the dark interior portion of the automobile roof to produce a dark background, while a partially reflective surface of the mirror reflects images from behind the vehicle to the driver's eyes. Thus, in this prior art mirror, the driver effectively sees a dimmed image of the view of objects behind the motor vehicle over a dark background image.
However, these rearview mirrors are incapable of producing a dimmed state in anything but dark conditions. Any ambient light will illuminate the interior roof of the vehicle causing the interior portion of the roof to be visible in the rearview mirror and thereby obstructing the image of the view behind the vehicle. Since prismatic dimmable mirrors cannot operate in a dimmed state without a dark region to which the high reflectance surface can be pointed, these mirrors can not be used as side mirrors located outside of the vehicle. Exterior side mirrors lack a dark region to which the high reflectance surface of the prior art prismatic mirror can be pointed.
Currently, no cost effective technique is utilized on large trucks, classes 6, 7 and 8 to operate a side mirror in a dimmed state. One prior art dimmable mirror technique, which is used in some luxury automobiles, positions an electronically controllable neutral density filter in front of a standard mirror. Two different technologies, electrochromic and dichroic liquid crystal, have been used to implement these electrically dimmable mirrors. However, each one of these technologies has major disadvantages which have limited their use. Solid state electrochromic technology has been used for dimming mirrors. The disadvantages of electrochromic mirrors is that they possess undesirable optical characteristics and are often too expensive for truck and many automobile applications.
Alternatively, dichroic liquid crystal ("LC") technology has been suggested for the manufacture of dimming mirrors that provide good optical characteristics at a reasonable cost. See in this regard U.S. Pat. No. 4,660,937, issued Apr. 28, 1987, to Richardson. However, existing prior art dichroic LC mirrors possess the undesirable characteristic of reverting to a low reflectance state when the electrical power to the mirror has been interrupted. In addition, as a consequence of conventional dichroic LC mirrors' power loss behavior, these dichroic LC mirrors do not meet the safety requirements of the U.S. National Highway Traffic Safety Administration ("NHTSA") for rearview mirrors. The NHTSA safety requirements are set out in 49 CFR .sctn.571.111 (1992). The relevant part of this regulation provides:
"All single reflectance mirrors shall have an average reflectance of at least 35 percent. If a mirror is capable of multiple reflectance levels, the minimum reflectance level in the day mode shall be at least: 35 percent and the minimum reflectance level in the night mode shall be at least 4 percent. A multiple reflectance mirror shall either be equipped with a means for the driver to adjust the mirror to a reflectance level of at least 35 percent in the event of electrical failure, or achieve such reflectance level automatically in the event of electrical failure."
Conventional dichroic LC mirrors do not comply with this NHTSA safety requirement because upon failure into a low reflectance state no means have been provided to adjust the failed mirror into a high reflective state as is required by the regulation.